1. Field of the Invention
This invention relates to upconversion systems and methods for stimulating a visible output from a laser or phosphor with an infrared input.
2. Description of the Related Art
Phosphors and other upconversion systems are important in display technology for television, radar, CRT, and instrumentation panels. The conventional television screen, for example, uses a screen coated with three different phosphors, typically corresponding to red, green and blue, and addressed by an electron beam. The electron beam generation apparatus is complicated and expensive, and the requirement for three separate types of phosphors adds to the overall complexity and cost.
Other upconversion mechanisms that convert infrared radiation to visible radiation have been known for many years in the field of phosphors. These upconversion systems emit visible radiation in response to an infrared pump beam, with the visible output considerably amplified with respect to the infrared input. They have been a topic of particular interest since the advent of inexpensive and high efficiency infrared diode lasers capable of providing the required pump beam to replace the costly electron beam generating apparatus.
A full color upconversion laser that is pumped by a single wavelength infrared source is discussed in U.S. Pat. No. 5,008,890 to McFarlane, "Red, Green, Blue Upconversion Laser Pumped by Single Wavelength Infrared Laser Source" and assigned to Hughes Aircraft Company, the assignee of the present invention. The laser employs a crystal of YLiF.sub.4 :Er5%, and is operated at a cryogenic temperature between 15.degree. and 120.degree. K. Most other investigations of upconversion lasers have similarly involved fluoride-based materials that required large scale cooling, e.g., Nguyen et al., "Blue Upconversion Thulium Laser", SPIE Vol. 1223-Solid State Lasers, 1990, pp. 54-63, and Hebert et al., "Visible CW-Pumped Upconversion Lasers", Proc. International Conference on Lasers '90, December 1990, pp. 386-393.
Investigations have been conducted to characterize the upconversion spectroscopic properties of erbium (Er) doped cesium cadmium bromide (CsCdBr.sub.3) crystals by Cockroft et al., in "Upconversion Fluorescence Spectroscopy of Er.sup.3+ Pairs in CsCdBr.sub.3 ", Journal of Luminescence, Vol. 43, 1989, pp. 275-281 and in "Dynamics and Spectroscopy of Infra-Red to Visible Upconversion in Erbium Doped Cesium Cadmium Bromide (CsCdBr.sub.3 :Er.sup.3+)", submitted to Physical Review B, Apr. 16, 1991. It was noted that an 804 nm wavelength infrared pump beam produced an emission peak at 414 nm, a 984 nm pump beam produced an emission peak at 493 nm, and that simultaneous 804 nm and 984 nm excitation gave strong emissions at 455 nm, 651 nm and 671 nm. Moreover, these investigations were conducted by pulse pumping the CsCdBr.sub.3 :Er.sup.3+ crystal at cryogenic temperatures ranging from 10.degree.-40.degree. K., as dictated by the state-of-the-art. Cockroft et al. are silent with respect to an upconversion laser system incorporating the CsCdBr.sub.3 :Er.sup.3+ crystal, and with respect to such a laser system or an upconversion phosphor being operable above cryogenic temperatures.
In general, the operation of an infrared upconversion system has been restricted to less than 140.degree. K. to reduce multiphonon relaxation of ion energy levels that are populated by the energy pooling of pumped ion pairs, and thus retain proper upper energy state laser populations. The requirement for cryogenic cooling to achieve upconversion characteristics in a laser system is known to significantly complicate the system, adding to its weight, bulk and expense. Therefore, there is a need for robust upconversion laser systems and phosphors which are operable above cryogenic temperatures to greatly expand the display application areas of the technology.